1. Field of Invention
This invention relates to an improved vending machine power switching apparatus, and particularly to vending machines and coin mechanisms having a plurality of relays for selecting which of a plurality of loads is to be energized.
2. Description of the Prior Art
Vending machines include a wide variety of coin operated machines such as cigarette, coffee, soda, candy and ticket dispensing machines, as well as, juke boxes, pinball machines and the like. Typically, vending machines employ relays for a wide variety of switching functions.
A relay is a commonly known electrical component which may be broadly defined as an electrically controlled device that opens and closes electrical contacts to effect the operation of other devices in the same or another electrical circuit. Typically, a relay incorporates an electromagnet, to which a controlling current is applied, which moves electrical contacts to switch the controlled current. As with any electrical components, the designer or manufacturer who employs relays in a system is faced with the task of minimizing the cost of the components used while still achieving the desired reliability and durability in the system.
Although alternative switching devices, especially semiconductor devices, are available and have been successfully used in vending machines, electromechanical relays are still preferred for many applications because the use of an electromagnet provides electrical isolation between the controlling and controlled currents, and because a single controlling current and electromagnet can control a plurality of circuits with a plurality of switching contacts. In the case of the common power-controlling semiconductor devices (such as SCR's, triacs and power transistors,) a separate isolation device (such as an opto-isolator) would be required and each power controlling device can open and close only one circuit. A further advantage of relays is that they are not subject to random turn-on due to random noise spikes.
One of the basic problems in designing vending systems with relays is the problem of contact deterioration or degradation due to arcing, fine metal transfer, and microscopic point welding which occurs when the relay contacts open or close a circuit in which current is flowing. For a general discussion of arcing and like phenomenon, see Frank S. Oliver, Practical Relay Circuits, Ch. 2 (1971). Thus, the principal rating of relays is current switching capacity.
The contact degradation problem is exacerbated where a relay is used to switch power to an inductive load. In conventional vending machines, relays are frequently used for switching current to inductive loads such as motors, solenoids and other relays. Some of the many functions of vending machines where power is switched to an inductive load are illustrated by the following U.S. Patents assigned to the assignee of the present invention: U.S. Pat. No. 3,792,766 (solenoid used in magnetic coin eliminator), U.S. Pat. No. 3,797,307 (solenoid retracts arrest pin), U.S. Pat. Nos. 3,814,115 and 4,367,760 (actuator operates slide plate for dispensing coins to be returned as change), U.S. Pat. No. 4,106,610 (actuators activate gates for directing coins through coin mechanism) and U.S. Pat. Nos. 4,234,070 and 4,458,187 (actuators dispense customer selected products).
Relay manufacturers have devoted extensive work over the years to produce contacts which, for a given size, are less prone to deterioration from switching current. Vending machine designers, concerned with long term reliability, have incorporated special protective circuitry to supress contact arcing. This response to the problem is not fully satisfactory where large numbers of relay are required because the provision of protective circuits for each relay and the cost for the relays used becomes quite expensive.